Gsa is a ubiquitous G protein a subunit which activates adenylyl cyclase upon stimulation of a variety of receptors including the PTH/PTHrP receptor (PPR). Both inactivating and activating mutations in Gsa have consequences in skeletal development. To determine the role of Gsa signaling during osteoblast development in vivo, we have conditionally deleted Gsa in early osteoblast precursors by mating Gsa-floxed mice with osterix promoter-Cre recombinase transgenic mice. Mice with early osteoblast-specific inactivation of Gsa (BGsaKO) have marked skeletal fragility and exhibit numerous fractures by postnatal day 1. Long bones from BGsaKO mice demonstrate severe reduction in trabecular and cortical bone. Specific Aim I will evaluate the physiologic effects of Gsa ablation in osteoblast precursors on osteoblast differentiation, bone formation and bone resorption in vivo. In Specific Aim II the focus will be to isolate bone marrow stromal cells from BGsaKO mice. These cells can then be used to probe the signaling pathways regulating osteoblast development, in particular to examine whether Gsa is required for the PKA-dependent stimulation of osteoblast-specific gene expression by PTH. [unreadable] [unreadable] [unreadable]